In a gantry of an existing X-ray CT (Computed Tomography) apparatus, a collimator is provided for a front face of an X-ray tube field. The collimator has a wedge for adjusting cutting of a soft ray and the intensity distribution of an X ray, and a slit mechanism which opens/closes in accordance with slice thickness at the time of a scan. The collimator forms a fan beam whose radiation dose is optimized, and irradiates a subject with an X ray.
In the case where the slit mechanism has a biaxial opening/closing mechanism in which right and left slits operate singularly, to suppress unnecessary irradiation of X rays generated before and after a scan, active collimation operation of controlling the width and position of an opening by varying the timings of opening the right and left slits to optimize the radiation dose can be performed. To execute the active collimation to reduce the dose of radiation to the subject, generally, a slit mechanism in an optical system has to have a biaxial opening/closing mechanism which can open/close two slits singularly. However, since the system is much more expensive than a normal optical system having no biaxial opening/closing mechanism in which two slits open/close, it is demanded to realize the active collimation operation by an inexpensive slit mechanism in which slits are opened/closed simultaneously.
According to one embodiment, an inexpensive high-performance X-ray CT apparatus with reduced radiation dose is provided.